This invention relates generally to semiconductor devices and, more particularly, to semiconductor devices having compensated buffer layers.
As is known in the art, semiconductor devices such as field effect transistors and monolithic microwave integrated circuits are often employed to amplify or process radio frequency signals. For example, a field effect transistor is often employed to convert D.C. power to radio frequency power by feeding an R.F. voltage signal to a gate electrode of the field effect transistor to thereby control the conductivity of an underlying drain-source channel of the field effect transistor.
Radio frequency performance from field effect transistors is dependent upon the quality of the crystalline structure of the semiconductor layers used to provide the field effect transistor. As is also known in the art, Group III-V semiconductor material systems such as systems employing gallium arsenide are often used to fabricate field effect transistors for amplifying or converting D.C. power to radio frequency and, in particular, microwave power. One technique used in the prior art to provide semiconductor layers for field effect transistors is to grow active regions directly over substrates of the Group III-V material. However, the crystalline quality of the substrates fabricated by any of the known methods is generally not suitable for fabrication directly thereon of high quality field effect transistors because in the crystal structure close to the surface of the substrate, unwanted crystalline defects such as hole and electron traps are present which can degrade the electrical properties of the device fabricated thereover. These traps can become ionized sites either accepting or emitting an electron. Thus, during operation of the field effect transistor, the electric field created by ionization of these traps will restrict the flow of electrons in the channel, an effect generally known in the art as "backgating" providing a concomitant loss in power.
One method known in the art for reducing this effect is to provide a buffer layer, comprising an epitaxially grown crystalline layer, intermediate the active regions of the semiconductor and the substrate. The buffer layer provides a high quality, high resistivity layer which shields or isolates the active region of the field effect transistor from the defects in the gallium arsenide crystal substrate. The buffer layer should preferably have a high resistivity and should also be relatively thick to adequately isolate or shield the active regions of the transistor from crystal defects present in the crystalline structure of the substrate.
Several methods have been suggested in the art to provide high resistivity buffer layers. These methods generally include the step of growing the epitaxial layer having a compensation dopant material introduced to maintain the high resistivity characteristics of the buffer layer. As shown in FIG. 1, Group III-V systems, for example, GaAs often have stoichiometric defects resulting from extra or interstitial As atoms. This type of stoichiometric defect generally referred to in the art as EL2 provides electron donor energy levels intermediate conduction and valence bands of the GaAs crystal. Also, certain impurities such as Si and S which are unintentionally introduced during growth of the GaAs layer provide shallow electron donor energy levels or energy levels just below the conduction band level of the crystal. Generally, this stoichiometric defect EL2, as well as, the impurities introduced during growth of the GaAs crystalline layer in the presence of an electron current flux provide donors of electrons and an electron current flow in the conduction band of the crystal. It is an object, therefore, to compensate for this electron current flow by providing a predetermined dopant concentration of an acceptor atom such as chromium which will provide in the presence of an electron current flux a hole current flow in the valence band of the crystal. This hole current flow in the valence band of the crystal compensates for electron flow in the conduction band of the crystal.
Generally, a dopant such as chromium is introduced during growth of the GaAs layer to provide acceptor energy levels to compensate for the donor energy levels. Chromium is a so-called deep level acceptor in GaAs. Chromium in GaAs provides an energy level which is intermediate the valence band energy level and the conduction band energy level of the GaAs. Chromium, when ionized accepts an electron from the valence band and provides a corresponding hole flow in the valence band of the crystal. Several problems occur with chromium dopant buffer layers, particularly when high concentrations of chromium are introduced into layers which are adjacent to the active layer of the field effect transistor. The rate of re-combination of electrons and holes between the valence band and intermediate deep energy level provided by the chromium may be lower than the rate of change of an injection current flux in the conduction band of the crystal. This lag in re-combination results in a net fixed negative charge of chromium ions in the crystal adjacent to the active layer. This fixed charge repels electrons in the channel of the device resulting in a loss of power. A second problem associated with chromium doping is that chromium has a tendency to slowly out-diffuse from the buffer layer into the active layer/buffer layer interface region resulting in a decrease in electron mobility and degraded device performance.